There is a long standing interest in using a control field to control a main field in an inductive device. For example, U.S. Pat. No. 4,210,859 describes a device comprising an inner cylinder and an outer cylinder joined to one another at the ends by means of connection elements. In this device the main winding is wound around the core and passes through the cylinder's central aperture. The winding axis follows a path along the cylinder's periphery. This winding creates an annular magnetic field in the cylinder's wall and circular fields in the connection elements. The control winding is wound around the cylinder's axis. It will thus create a field in the cylinder's longitudinal direction. The core's permeability is changed by the action of a control current applied to the control winding. Because the cylinders and the connection elements are made of the same material, the rate of change of permeability is the same in both types of elements. Consequently, the magnitude of the control field must be limited to prevent saturation of the core and decomposition of the control field. As a result, the control range of this inductor is limited, and the device, in U.S. Pat. No. 4,210,859, has a relatively small volume that limits the device's power handing capability.
Other devices include controlled permeability of only part of the main flux path. However, such an approach dramatically limits the control range of the device. For example, U.S. Pat. No. 4,393,157 describes a variable inductor made of anisotropic sheet strip material. This inductor comprises two ring elements joined perpendicularly to one another with a limited intersection area. Each ring element has a winding. The part of the device where magnetic field control can be performed is limited to the area where the rings intersect. The limited controllable area is a relatively small portion of the closed magnetic circuits for the main field and the control field. Part of the core will saturate first (saturation will not be attained simultaneously for all parts of the core because different fields act upon different areas) and this saturation will result in losses generated by stray fields from the main flux. Partial saturation results in a device with a very limited control span.
Thus, the prior art lacks a means to control permeability in a core for substantial power handling capability without introducing considerable losses. The shortcomings of the prior art effect all inductive device geometries, and in particular, curved structures made of sheet strip metal because considerable eddy currents and hysteresis losses occur in these types of curved structures.